Process for the preparation of alkaline chromates from chromium minerals

ABSTRACT

A process for the production of alkaline chromates by means of oxidative disaggregation in a reactor of substances containing trivalent chromium compounds in admixture with alkali. The oxidative disaggregation is carried out in dry phase by moving the mixture within the reactor while heating the mixture by indirect heat exchange in the absence of combustion gases and under mechanical stress, and feeding to the interior of the reactor oxidizing gas. The mixture is heated to a temperature of between 500° and 1500° C. The oxidizing gas is fed with an oxygen concentration in a range between 8 and 100%. Gases leaving the reactor are used for the acidification of an aqueous solution of alkaline chromates which is then dried and supplied to the reactor. The reactor is a rotating tubular reactor and the mixture moves continuously through the rotating reactor.

The present invention concerns the oxidative disaggregation of chromiumminerals, that is to say the preparation of hexavalent chromiumcompounds by means of the oxidation of trivalent chromium compounds and,in particular, of minerals containing trivalent chromium.

The extraction of trivalent chromium from the natural minerals whichcontain it (minerals which will henceforth be referred to with thegeneric term "chromite") involves oxidizing trivalent chromium presentin the minerals to hexavalent chromium and then extracting it, by meansof subsequent leaching, in the form of hexavalent chromium solublesalts.

Conventional methods for the oxidative disaggregation of chromiteinvolve finely grinding the mineral, which is then oxidized in thepresence of sodium carbonates and/or sodium hydroxide or of otheralkaline metals, at a temperature of between 600° and 1500° C.

In addition, thinning materials and sometimes oxidants are added to themixture. Generally ferrous oxides and leaching residues are used asdiluents.

The high temperatures necessary for the oxidation reaction are obtainedby heating the mixture directly, that is to say by putting the mass tobe disaggregated in direct contact with the flame and with thecombustion products of the burner usually diluted in the atmospheric airnecessary for oxidation.

Various working methods or compositions of the mix have been proposedwith the aim of improving process yields.

For example, German patents Nos. DE-25 57 403 and DE-26 07 131 concernthe disaggregation of chromite minerals lean in chromium butparticularly rich in silica; German patent No. DE-25 42 054 proposes amulti-phase process which, in its preferential form, foresees three hotoxidation cycles (roasting), with the aim of raising the yields of thetransformation of the CR₂ O₃ of the chromite into hydrosoluble chromateto values to between 70 and 85%.

European patent No. EP-A-027 868 describes feeding furnaces with amixture of minerals pelletized in an aqueous liquid, obtained inpractice by the use of wet leaching residues.

The main disadvantage with all these known processes described above isthe difficulty of obtaining a concentration of O₂ in the oxidizing gaseswhich is sufficient for the complete oxidation of the trivalent chromiumpresent in the mineral, since the oxygen present in the combustion fumesenriched with air is usually no more than about 8-10% of the total.

In addition, the fact that both the combustion and dilution gases arebrought into direct contact with the mixtures leads to the entrainmentof the particles from the furnace. These particles must be separated,with consequent and not indifferent ecological damages, since theycontain hexavalent chromium also.

In order to improve the characteristics of the atmosphere inside thefurnace, attempts have been made to increase the oxygen content of theoxidizing gases. For example, Japanese patent No. 75905 (Nippon K. K.),which describes the admission of an oxygen-rich gas under the flame of adirectly heated rotating tubular furnace. Such a solution leads howeverto an increase in the entrained particles and in the formation of ringsof fouling without any valuable yields improvements.

In an alternative process, known since the beginning of the century, (wecite for example, German patents No. DE-163814 and No. DE-171089), theoxidation reaction of chromium minerals is carried out at a relativelylow temperature (400°-700° C.) by using low flux mixtures, generallyobtained thanks to the presence of high quantities of alkalinehydroxides in the mixture. Oxidation is usually assisted either byinjecting oxygen-containing gases into the melting bath or by theaddition of oxygen donors (such as nitrogen acid alcohol salts,manganese oxides, lead oxides and the like) to the mixture.

A further problem arising from the known technique is due to the factthat the monochromate which forms during the reaction melts at theroasting temperature of the mineral.

It has been found that the melted monochromate initially is dispersed inthe material present in the reactor, and is "absorbed" by it. When acertain weight percentage of monochromate (Na₂ CrO₄) of the total weightof material loaded into the reactor is surpassed the gangue of thechromite can no longer "absorb" and retain the melted monochromate,which comes into contact with the reactor, forming rings of fouling (inthe case of tubular furnaces), with all the accompanying disadvantages.

The chromite mineral usually contains a quantity of chromium oxides suchas will give a weight of monochromate greater than the maximumpercentage "absorbable", which for the known processes in dry phase isnot usually higher than 40%.

This fact has caused various attempts to find different types ofsolutions so as to be able to work with economically acceptableconversion yields of chromium and alkaline compounds.

A first solution, which is the one most widely adopted, consists inadding to the mixture to be subjected to oxidative disaggregation one ormore "thinning" materials with no or a very low chromium content, whichcan thus retain or "absorb" the melted monochromate.

For this purpose, use is often made of the resultant earths from aprevious oxidative disaggregation.

This solution has the disadvantage of having to treat each time a massof material much larger than the ideal one, which consists of onlyminerals and alkali.

A second solution is described in the U.S. Pat. No. 3,963,824. In thistechnique the ground chromite mineral is dispersed in a bath of low fluxalkaline salts and indirectly heated in a reactor, under agitation andwith the injection of oxygen. The main disadvantage of this techniqueconsists in the excessive consumption of alkaline salts, which must bepresent in a ratio to the mineral varying between 5:1 and 20:1, whichmakes this technique substantially unsuitable for application toindustrial processes because of the high costs it involves.

According to a further technique, described in patent US-3,295,954 a"binary" mixture, that is to say of only chromite minerals and alkali,is subjected to oxidative disaggregation while the mixture is at rest.For this purpose the mixture is prepared in cakes, which are placed inspecial containers and dragged through an indirectly heated furnace,inside of which an atmospheric air current flows. In this way it ispossible to avoid the escape of melted monochromate from the mineral,even in the absence of thinning materials, thus obtaining atransformation yield of the chromium present in the mineral of about90%.

The disadvantage of this technique lies in the fact that it requires themixture to be in a state of absolute rest during oxidation. Continuousreactors such as rotary furnaces are explicitly excluded from thistechnique, as they would give rise to the above-mentioned phenomena ofthe formation of foulings and rings. Therefore it is easy to see theuneconomical nature of this technique also, as well as its difficultpractical realization.

The present invention aims at a process which allows the oxidativedisaggregation of materials containing trivalent chromium compounds,under controlled conditions, with high yields and without the formationof foulings and rings.

A second aim of the invention is to carry out such oxidativedisaggregation by means of a process which allows the chromium compoundscontained in the said minerals containing trivalent chromium to beconverted in a short time.

A third aim of the invention is to be able to disperse the combustiongases directly into the outside atmosphere without the need for plantsfor purifying fumes from the chromium residues.

All of these aims have been achieved by means of the present invention,which teaches a process for the production of alkaline chromates bymeans of oxidative disaggregation of minerals and/or substancescontaining trivalent chromium compounds in the presence of alkali,characterized in that said oxidative disaggregation is carried out indry phase by heating said material in a controlled environment and witha pre-established oxygen percentage adjustable according to the desireddwell time of the material in said reactor.

By means of this process a significant increase in the transformationyield is obtained and there is a surprising increase in the kinetics ofthe reaction, with a consequent reduction in firing time and greatbenefits in terms of furnace productivity.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described, in greater detail, with referenceto the appended drawings which are to be considered in a purelyillustrative and no limitative way, in which:

FIG. 1 shows a longitudinal sectional view of a preferred embodiment ofa plant for practicing a method according to the invention;

FIG. 2 shows a diagram illustrating the phases of the process accordingto the invention;

FIG. 3 shows a graph, on which are marked the transformation yields bothof soda and of chromite in relation to the time for differentconcentrations of oxygen, using the process according to the invention.

The process according to the invention involves oxidativedisaggregation, that is to say that the material containing trivalentchromium compounds is initially ground to a pre-established granulometryand then mixed (preferably in dry phase) with alkali and, if necessary,with thinning materials, which have also been ground to the desiredgranulometry.

The mixture thus obtained can be pre-heated according to knowntechniques (for example, by the use of the hot gases from the plant),and then fed into a reactor where the oxidation of the mineral, leadingto the formation of alkaline chromates, takes place.

According to the principles of the invention, the heating means aredistinct from the means used for feeding the oxidation gases, and theoxygen needed for this reaction is supplied by feeding the gasescontaining different, pre-established and adjustable concentrations ofoxygen, in a controlled way, to the mixture. In other words, unlikeknown processes, the oxidation phase takes place in a controlledenvironment in the absence of combustion products of burners, which intraditional processes supply the system with both the heat and theoxygen necessary for the reaction. Thus, the reaction is carried out indry phase by heating and moving the mixture within the reactor in theabsence of combustion gases and under mechanical stress.

At the end of the oxidation reaction the "frit" obtained is leached inthe know manner in order to extract as aqueous solutions the alkalinechromates; object of the present invention.

The heating of the mixture as above described allows the verifying ofthe composition of the oxidizing gases fed to it by adjusting theconcentrations of O₂ in relation to the composition of the mixture, thereaction temperature and the kinetics desired introducing gases withoxygen concentrations ranging from 8% to 100%.

In addition to the adjustment of the oxygen concentration the reactioncan be controlled by adding oxidizing compounds to the mixture.

Furthermore, the process according to the invention, as will be clearlyseen from the examples cited below, has proved to be particularlyadvantageous in the case where the reactor is fed with gases containinga volume of oxygen superior to that contained in air (about 21%), andparticularly when the reactor is fed with substantially pure and heatedoxygen.

The heating of the mixture in a controlled environment keeps thecombustion products from the burners separate from the oxidizing gasesand thus avoids entrainment of the particles in the gases originatingfrom the heating of the reactor. Consequently, the expensive separationand purification plants indispensable to traditional processes are nolonger necessary.

The leaving combustion products free from particles, in particular fromchromium, can be therefore directly sent to exchanging means forrecovering the heat and then directly to the atmosphere.

The gases resulting from the oxydation reaction in the case in whichalkalis present in the mixture include carbonates and/or bicarbonatescontain carbon dioxide generated from the decomposition of saidcarbonates/bicarbonates.

A concentration of CO₂ even higher than 90% can be obtained in the gasesleaving the reactor by opportunely regulating the flow rate and thecomposition of the oxidizing gases.

This result has been achieved by using suitable means which prevent thegas dilution inside the rector.

The carbon dioxide thus obtained, after being appropriately washed andcooled, can be advantageously directed to the acidification process toprovide carbonation of the aqueous solution of alkaline chromatesobtained by leaching the oxidized mixture.

At this point it should be noted that the present invention cantherefore allow reactor outlet gases to be obtained with a high enoughconcentration of carbon dioxide to allow them to be fed directly to themeans of carbonation of the chromate solution, without having first toconcentrate the carbon dioxide.

Furthermore, the volume of carbon dioxide obtained is such that it isentirely sufficient for the needs of the process of transformation ofthe chromate into alkaline dichromate for carbonation. The reactorgenerates high concentrations of carbon dioxide and thus obviates theneed for special carbon dioxide production plants.

Another particularly advantageous aspect of the process according to theinvention is the fact that the gases from the reactor outlet areanalyzed, thus making it possible to adjust the conditions of reactionand maximize the production cycle.

In addition, by appropriate adjustment of the oxidation gases, thisinvention allows more or less complete conversion of alkali intoalkaline monochromates (see FIG. 3), and this is reflected by the natureof the compounds resulting from the working of the chromite.

In fact, contrary to what happens during the known processes ofoxidative disaggregation in the absence of calcium compounds (or whenthe quantity of such compounds in the mixture is limited), the resultantearths obtained after leaching surprisingly do not have magneticcharacteristics.

This means that the chemistry of the firing and oxidation phases differssubstantially from what is so far known, insofar as the presentinvention makes possible the complete transformation of the carbonateinto chromate.

This particular result also makes it possible to control the extent ofthe use of alkali in the process of firing the mixture, in relation tothe presence or absence of magnetic characteristics in the resultantearths.

On this subject, it must be stressed that obtaining resultant earthswith magnetic characteristics indicates that the describedtransformation has not been completed, possibly improving plantcapacity.

A series of preliminary tests will now be described, which were carriedout in a laboratory muffle furnace. Their results form an importantbasis for finding new and important characteristics of the process, andconsequently of the plant according to the invention.

The chromium mineral mentioned in the following examples is a chromitewith the following composition: Cr₂ O₃ 46.2%, FeO 27.1%, Al₂ O₃ 15.9%,MgO 9.7%, SiO₂ 1.0%.

EXAMPLE 1

A mixture, made up of 100 parts of chromium mineral, 66 parts of sodiumcarbonate and 120 parts of dried leaching residues derived from previousdisaggregations, is roasted at 1050° C. for 30 minutes in a laboratorymuffle furnace.

Numerous tests are made, changing the oxygen content of the gaseousmixture present in the furnace.

The yields in terms of the transformation of the chromium present in themineral into sodium chromate, that can be leached from the reactionmixture vary as follows:

    ______________________________________                                               O.sub.2 % Vol.                                                                        yield                                                          ______________________________________                                                4      52%                                                                    8      70%                                                                   10      78%                                                                   21      83%                                                                   100     95%                                                            ______________________________________                                    

EXAMPLE 2

A mixture, made up of 100 parts of chromium mineral, 66 parts of sodiumcarbonate, 31.5 parts of lime and 88.5 parts of dried leaching residuesderived from previous disaggregations, is roasted at 1050° C. for 30minutes in a laboratory muffle furnace.

Tests are made, changing the oxygen content of the gaseous mixturepresent in the furnace.

The yields in terms of the transformation of the chromium present in themineral into sodium chromate that can be leached from the reactionmixture vary as follows:

    ______________________________________                                               O.sub.2 % Vol.                                                                        yield                                                          ______________________________________                                                7      82%                                                                     10.5  87%                                                                   21      90%                                                            ______________________________________                                    

EXAMPLE 3

A mixture, made up of 100 parts of chromium mineral and 48 parts ofsodium carbonate, is roasted at 780° C. for 3 hours in a laboratorymuffle furnace.

Two tests are made: the first in the air and the second in pure oxygen.

The yield in terms of the transformation of sodium carbonate intochromate passes from 71.9 to more than 99%.

EXAMPLE 4

A mixture made up of 100 parts of chromium mineral, 54 parts of sodiumcarbonate and 46 parts of dried leaching residues derived from previousdisaggregations, is roasted at 960° C. for 10 minutes in a laboratorymuffle furnace.

Two tests are made: the first in the air and the second in pure oxygen.

The yield in terms of the transformation of sodium carbonate intochromate passes from 88.2 to more than 99%.

EXAMPLE 5

A mixture, made up of 100 parts of chromium mineral and 33 parts ofsodium carbonate, is roasted at 900° C. for 10 minutes in a laboratorymuffle furnace.

Two tests are made: the first in the air and the second in pure oxygen.

The yield in terms of the transformation of sodium carbonate intochromate passes from 88.7 to more than 99%.

EXAMPLE 6

A mixture, made up of 100 parts of chromium mineral and 43 parts ofsodium carbonate, is roasted in pure oxygen at 950° C. for 10 minutes ina laboratory muffle furnace.

Under such conditions, the yield in terms of the transformation ofsodium carbonate into chromate is more than 99%.

The frit is leached and 100 parts of the dried residue are mixed with 30parts of sodium carbonate and once again roasted in oxygen, this time at1050° C. for 1 hour.

Under these conditions, the yield in terms of the transformation of theCr₂ O₃ present in the mineral into sodium chromate rises to more than95%.

The final residue contains just 3.7% of Cr₂ O₃.

The data arising from the above-mentioned examples show how an increasein the transformation of chromium itself corresponds to the increase inthe concentration of oxygen in the environment in which thetransformation reaction of chromium takes place. These yields reachalmost 100% in the case where pure oxygen is present in the reactionchamber.

The invention will now be further described by means of the followingexamples, which are to be considered purely as examples of the presentinvention and in no way restrictive.

EXAMPLE 7

A mixture, made up of 100 parts of chromium mineral, 52.5 parts ofsodium carbonate and 15 parts of dried leaching residues derived fromprevious disaggregations, is continuously sent to a pilot roastingplant. The said plant consists of an externally heated unfettledrotating tubular reactor in special alloy, having the followingdimensions: diameter--250 mm; heated length--3000 mm. The mixture is fedat a rate of 7.5 kg/hour and the internal temperature of the reactor ismaintained at 980° C. With a countercurrent air flow, the yield in termsof the conversion of sodium carbonate into sodium chromate is just over96%.

EXAMPLE 8

A mixture, made up of 100 parts of chromium mineral, 52.5 parts o sodiumcarbonate and 15 parts of dried leaching residues derived from previousdisaggregations, is continuously sent to the plant described in theabove example. Maintaining the internal temperature of the reactor at980° C., but sending in a countercurrent flow of pure oxygen, a feedrate of 28 kg/hour gives yields in terms of the conversion of sodiumcarbonate into sodium chromate of more than 98%. The residues obtainedafter leaching the reaction product do not have any magneticcharacteristics. The gas at the reactor outlet has the followingindicative dry composition: CO₂ 88%, O₂ 11%, N₂ 1%.

EXAMPLE 9

A mixture, made up of 100 parts of chromium mineral, 26.5 parts ofsodium carbonate, 41 parts of sodium bicarbonate and 15 parts of driedleaching residues derived from previous disaggregations, is continuouslysent to the plant described in Example 7, maintaining inside the reactora temperature of 980° C., by sending a countercurrent pure oxygen flow,at the rate of 29.5 kg/hour, conversion yields of sodium carbonates intosodium chromate of more than 98% are obtained.

The residues of the reaction product obtained after leaching do not haveany magnetic characteristics. The gas at the reactor outlet has thefollowing indicative dry composition: CO₂ 90%; O₂ 9%; N₂ 1%.

EXAMPLE 10

A binary mixture, made up of 100 parts of chromium mineral and 57.3parts of sodium carbonate is continuously sent to the plant described inExample 7. The mixture is fed at a rate of 28 kg/hour. The temperatureinside the reactor is maintained at 990° C., and with a countercurrentflow of pure oxygen a yield is obtained in terms of the conversion ofsodium carbonate into sodium chromate of more than 97%.

EXAMPLE 11

A binary mixture, made up of 100 parts of chromium mineral and 57.3parts of sodium carbonate is continuously sent to the plant described inExample 7, in which the temperature inside the reactor is maintained at990° C. With a countercurrent flow of pure oxygen, pre-heated to 800°C., a yield is obtained in terms of the conversion of sodium carbonateto sodium chromate of more than 97%, with stay times of the mixturebeing roasted of less than 10 minutes. Under such conditions, theconversion of the Cr₂ O₃ originally present in the mineral into sodiummonochromate is more than 85%.

Oxidation in a controlled oxygen content atmosphere gives, therefore,under the same working conditions, the desired transformation yield withstay times short enough (see FIG. 3) not to allow the melted part toseparate from the "inert" part.

In other words, the control of the O₂ content of the oxidizing gases andof the other reaction parameters allows working with a very highreaction speed, enriching in a short time the gangue of the mineral withmelted monochromate, and discharging it all as soon as the conversionhas reached the desired percentage, before rings and foulings can formin the reactor.

As described in the Examples from 7 to 11 the preferred reactor is madeof an indirect heating rotating furnace, provided with means forcontinuously moving the mixture during its flowing inside the tubularreactor. The oxidizing gases are preferably fed in countercurrent to themixture flow through the reactor.

The special nature of the process according to the invention is visuallyapparent also in the product thus obtained.

It has in fact been observed that the mineral containing themonochromate which has undergone an oxidation process according to theinvention comes out from the tubular reactor in the form of almostglobular and, for the most part, porous particles.

This physical aspect of the roasted mineral is particularly advantageousinasmuch as it allows the particles to be sent to a quencher while stillhot, without too much powder, and without the necessity for crushingthem previously, therefore facilitating the leaching of the materialitself.

The quenching also takes place in more uniform and less violent manner,with respect to the commonly used plants.

By the use of a binary mixture, as foreseen in the preferred embodimentof the process according to the invention, the weight ratio betweenmonochromate content and the gangue or inert portion of the mineral ishigher than the rate of 1:1, with obvious advantages for its subsequenttreatment.

From the graph shown in FIG. 3 it will be very clear how the processaccording to the invention gives marked increases in the yields of sodaand of chromium mineral, and at the same time a reduction in firingtimes.

The data shown refer to an oxidation reaction carried out at atemperature of 1000° C. on a binary mixture.

With reference to FIG. 1, a diagrammatic description will now be givenof a preferred plant for the realization of a process according to thepresent invention. As has already been mentioned, such plant has arotating tubular reactor 1, provided upstream with means 2 for feedingthe mixture, and with means 9 for feeding the outlet gases from thereactor to the heat exchanger and/or to the means for recovering carbondioxide and/or for analyzing the gases (not shown). At its downstreamextremity, it is provided with means 3 for conveying the oxidizedmixture to the leaching means (not shown) and with means 4 for feedingoxidizing gases in countercurrent to the flow of the mixture; the plantalso comprises sealing means 17 to prevent the gas dilution inside thereactor.

The reactor 1 is mounted inside the structure 5, generally in refractorymaterial, which forms the heating chamber of the furnace. The heatingchamber 5 is provided with means, shown schematically with referencenumber 6, for the heating of the tubular reactor 1. The heating means 6are known means such as, for example, burners above and below thetubular reactor.

On the upper part of the heating chamber 5 means 7 are provided fordischarging the combustion gases of the heating elements 6. Thesetechnically known means include valve means 8 for controlling the draftof the furnace 5.

It is evident that the plant according to the present invention allowsthe combustion gases from the heating elements 6 to be fed to the heatrecovery means (not shown), as they are completely free from chromiumparticles, and that the refractory material of the heating chamber 5,being placed outside the reactor, is insulated from the mixturecontaining the chromium compounds, thus avoiding any contamination ofthe furnace by the latter and making maintenance operations easier.

FIG. 2 is a schematic diagram showing a preferred embodiment of a plantfor practicing a method according to the present invention, and thevarious working phases.

With reference to the said drawing, the chromite, the alkali and thethinning material, if any, are fed to the means 10 for grinding and/ormixing and subsequently to the tubular reactor 11, which issimultaneously heated by combustion gases according to the methoddescribed above. The mixture is thus set in direct contact with theinner walls of the tubular reactor 11, which acts as a means fortransmitting heat and which, unlike traditional furnaces, does not havea layer of refractory material on its inner walls.

The oxidizing gases are fed into the reactor 11 along the line 12, in adirection which is countercurrent to the flow of the mixture: in thisway the gases at the outlet of the reactor, rich in carbon dioxide (asdescribed in the above examples), can be directly passed by the line 13to the means 14 for the acidification of the solution of alkalinemonochromate, which enters through the means 15 for leaching of theoxidized mixture.

The alkaline bicarbonate formed during the said acidification processmay be advantageously recycled, after appropriate treatment, to themixing means 10.

The diagram shows a heat exchanger 16 which can be used to recover heatfrom the gases of the reactor's heating burners. As shown, the gasescoming from the exchanger are directly discharged into the outsideatmosphere, as they are free from chromium.

As already underlined, the particular embodiment of the plant justdescribed must not be considered as limitative of the invention itself,the fundamental idea of the invention being able to take concrete formin quite different embodiments also.

For example, it is possible to think of an embodiment in which theheating of the oxidative reactor takes place inside the same by means ofelectrical resistances of the known type. In this case also theoxidation of the trivalent chromium takes place in a controlledenvironment with the oxidizing gases, which are capable of interactingentirely with the mineral to be oxidized.

We claim:
 1. A process for the production of alkaline chromates by meansof oxidative disaggregation in a reactor of substances containingtrivalent chromium compounds in admixture with alkali, comprisingcarrying out said oxidative disaggregation in dry phase by moving saidmixture within said reactor while heating said mixture by indirect heatexchange in the absence of combustion gases and under mechanical stress,and feeding to the interior of said reactor oxidizing gas with an oxygenconcentration comprised in a range between 8 and 100%.
 2. A processaccording to claim 1, wherein said mixture is heated to a temperature ofbetween 500° and 1500° C.
 3. A process according to claim 1, wherein theoxidizing gas is fed with an oxygen concentration greater than 21%.
 4. Aprocess according to claim 1, wherein pure air is fed as said oxidizinggas.
 5. A process according to claim 1, wherein substantially pureoxygen is fed as said oxidizing gas.
 6. A process according to claim 1,wherein oxidizing compounds are added to said mixture.
 7. A processaccording to claim 1, wherein the alkali is selected from the classconsisting of sodium carbonates, bicarbonates, sodium hydroxide andtheir mixtures.
 8. A process according to claim 7, wherein gases leavingthe reactor are used for the acidification of an aqueous solution ofalkaline chromates.
 9. A process according to claim 1, wherein saidmixture is indirectly heated by means of burners.
 10. A processaccording to claim 1, wherein at least one of said mixture and saidoxidizing gas is preheated.
 11. A process according to claim 1, whereinsaid reactor is a rotating tubular reactor and said mixture movescontinuously through said rotating reactor.
 12. A process according toclaim 1, wherein said reactor has unfettled walls and said mixture isheated by heat applied to the exterior of said unfettled walls.
 13. Aprocess according to claim 1, wherein said mixture initially consistsessentially of said trivalent chromium compounds and said alkali.